Continuous digester including mechanical defibering means therein



Jan. 17, 1967 T 3,298,903 CONTINUOUS DIGESTER INCLUDING MECHANICAL v DEFLBERING MEANS THEREIN Filed March 11, 1964 Illlllllllllllll) IIIIIIIJIIIIIIIII INVENTOR.

United States Patent O 3,298,903 CONTINUOUS DIGESTER INCLUDING MECHANI CAL DEFIBERING MEANS THEREIN 1 Robert H. Hart, Beloit Wis., assignor. to Beloit Corporation, Beloit, Wis., a corporation of Wisconsin.

Filed Mar. 11, 1964, Ser. No. 350,964

4 Claims. (Cl. 162-236) The present invention relates to a continuous 'digesterchemical pulping method and system, and more particularly to improvements in continuous chemical pulping processes and systems wherein there is a closer continuity between so-called digestion and the so-called pulping procedures by virtue of cont-rolled conditions for the cooking and digestion such that the individual fibers ultimately freed in the process are subjected to closer'toler ances with respect to the overall chemical treatment and/ or de-lignification resulting therefrom. i

In a typical chemical pulping process, such as-the 'kraft process, active pulping ingredients such as sodium hydroxide and sodium sulfide are used to treat the wood material usually consisting of wood chips. The dissolved organic constituents in the spent pulping liquors are burned for steam generation, and the inorganic pulping chemicals are recovered and reused. While the mechanics of such chemical pulping processes are reasonably well understood, heretofore it has been difficult to provide a suitable continuous process for pulping because ofthe extremes in process conditions which havebeen observed for the treatment of wood chips or the; like source materials in obtaining the pulp. Although the instant invention is not limited to the kraft pulping process, and may be readily adapted to any essentially chemical pulp ing process (as contrasted to an essentially mechanical pulping process), in many instances the instant invention will be described using the kraft process as typical, although the skilled worker in the art will readily recognize that other chemical processes can be used to substantially equal advantage.

The instant invention provides an improved continuity in the overall process, in that the wood chips are subjected to a temperature-time-pressure-chemical concentration relationship such that there is developed a predetermined gradient or variation in the speed of chemical reaction, whereby the speed of chemical reaction on the chemicals of the individual fibers actually decreases from the outer periphery to the inner periphery of any given chip or fiber bundle or particle in the digestion-pulping process. 'In general, it has been thought in the art heretofore that the only practical way to obtain uniform cooking conditions involved initial uniform saturation or impregnation of the chips with the cooking liquor under relatively moderate conditions, so as to avoid rather than effect a reaction gradient in speed of chemical reaction from the outside to the inside of the chip. The preferred notion was that uniform speed of chemical reaction throughout the entire chip was preferred. Experts, however, in this field are in considerable disagreement as to a number of theories as well as practical concepts and, although it may very well be quite desirable to efiect simultaneous and substantially identical cooking conditions throughout the entire body of each chip for uniform cooking, it must be appreciated that as one attempts continuous digestion and/or pulping in any real sense of the word, one starts to depart from this fundamental concept and it is apparent that according to many of the experts in the field difficulties will necessarily result.

The instant invention, however, is based upon not only the exact opposite of this type of thinking, but a practical method and system for employing such opposite thinking. In other words, the instant invention is based upon the concept-of creating a chemical reaction gradient from 3,2983% Patented Jan. 17, 1967 2 the exterior to the interior ofthe chips and using this gradient to free individual peripheral. fibers from the chip at a rapid rate, undercontrolled conditions of rapid cooking, in such a way that (1) the fibers which are initially subjected to the most active chemical reaction at the periphery of the chips will berernoved very quickly from the scene of active chemicalreaction and (2) the next inwardlayer of fibers will then become the outer or-peripheral layer of fibers on the chip and these fibers will then go through the procedure of (1) just described. Inessence, the concept involves using conditions which might heretofore have been thought to be too severe in that they would have inherently caused over-cooking of the initially chemically-attacked fibers at the periphery of the chip; but in the practice of the instant invention the cooking conditions involve certain movement and/or agitation of the chips such that the initially chemically-attacked fibers will respond first by weakening of their bonds to the chip and thus will be shaken loose easily under relatively mild conditions (in that-such conditions differ substantially from the concept of mechanical pulping), and the free fibers thus separated are quite rapidly removed from the sceneby a stream of liquor which drives the same through what constitutes assubstantially freed-fiber size restriction. In this way'the freed fibers are driven away from the scene of the greatest chemical activity, being driven through such restriction and then physically separated from a substantial quantity of the free liquor so that subsequent cooking conditions, if any, to which these fibers are subjected will be readily controlled and the fibers being in this condition essentially all of the same size and chemical content and characteristic will cook uniformly. In contrast, the chips or other particles which have not yet been reduced to freed-fiber size or condition will be rapidly recirculated through the scene of greatest chemical activity, so that peripheral fibers thereon will be repeatedly subjected only briefly to the chemical attack which weakens the bond and then permits the fibers to be freed and thus taken out of the scene of chemical action, or at least out of the scene of the major or more drastic chemical action, while the chip or particle which is constantly furnishing new, firmly bonded peripheral fibers to the scene of maximum chemical action will be continuously maintained therein or recirculated therethrough in such a way that there is a more or less continuous succession of maximum chemical action at the periphery of the chip followed by rather rapid freeing of the fibers thus acted upon at the periphery of the chip and exposure of new fibers at the periphery of the chip. In this continuous process the conditions are controlled to the extent that each fiber, as it were, in its original condition bonded to a bundle of fibers or the periphery of the chip will be subjected to substantially the same average amount of chemical action, but during a relatively shorter period of time than would ordinarily be used in most prior art attempts at continuous processes and, of course, for a substantially shorter period of time than is employed in known batch processes.

From the foregoing it will be apparent that it is an important object of the instant invention to providean improved continuous digester-pulping chemical process "and system.

Other and further objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed disclosure thereof and the drawings attached hereto and made 'a part hereof.

On the drawings: FIGURE 1 is essentially a schematic view of a digester-pulping system embodying the instant invention;

FIGURES 2 and 3 are essentially schematic views of i the digester element of the system shown overall in FIG- 3 URE 1, with FIGURES 2 and 3 showing modifications thereof, which are embodiments of the'invention; and

FIGURE 4 is a fragmentary sectional detail view of a type of freed-fiber size restriction which restriction construction is shown diagrammatically in FIGURES 1, 2 and 3 and designated by the reference numeral R in each case, although FIGURE 4 is a section of the element R shown in FIGURE 2, taken generally vertically through the center shaft thereof.

- As shown on the drawings:

The overall system of FIGURE 1, indicated generally by the reference numeral 10, contains a digester 11, discharge zones or elements R, R (which will be described in detail hereinafter) and a physical separation zone or element 12 which is shown only diagrammatically, but may have a typical construction of a known liquor transfer press. With respect to conventional construction of individual elements, attention is directed to various prior art disclosures which include Wells US. Patent No. 2,466,290, Obenshain Patent No. 2,789,051 and Armstrong and Nolan Patent No. 3,096,234, which show individually or collectively various elements that may be used in the practice of the instant invention at various locations and it is for this reason that many of the known commercially available individual elements employed in the practice of the instant invention are represented only schematically herein.

In following the cellulosic components through the system of FIGURE 1, it will be appreciated that chips C enter the top of the digester 11 via a conventional rotary pressure type feeder 21, and the chips pass from the feeder on down to the liquor level L-1 indicated in dotted lines within the digester 11. The liquor in the digester 11 is constantly being circulated therein and as soon as the chips in the embodiment of the system have ab sonbed enough aqueous liquor to sink into the moving recirculating streams of liquor within the digester, the previously mentioned initial chemical action takes place and the peripheral fibers are rapidly and continuously removed from the chip merely by virtue of the mild agitation effect that is imparted to the chips moving about in the liquor streams by the rotary action of the restriction members R. In this respect, attention is directed briefly to FIGURE 4, in that it is seen that the devices R, R, etc. are all essentially the same in construction as the device R specifically referred to in FIGURE 4, in that these devices R are in each case mounted on rotary driven shafts 30 preferably mounted on pair's of'bearings 31. and 32 for the cantilever type mounting here employed, with the shaft being concentrically mounted within a liquor stream removal housing 33 of generally cylindrical diameter but having a take-off line 34, which will ultimately feed to a main header indicated at 35:: in FIGURE 1, 35b in FIG- URE 2 and 350 in FIGURE 3, as feeding to the physical separator 12. The rotary shaft 30 for the restricting zone or element R mounts a pump impeller type of head shown generally in section at 36 in FIGURE 4 and comrestriction 37. It will be appreciated that the annular rib 38 is preferably fixed and merges'r'el'atively smoothly with the digester wall 39 so as to avoid dead areas, whereas the impeller disk 36x is moving relative to the annular ri b 38 continuously so as to effect a more or less continuous self-cleaning of the gap 37. As previously mentioned, the impeller blades 36a, 36b, 360, etc. also serve to drive solid particles countercurrent to the flow of liquid into the gap 37 and thus drive these particles or unreduced chips away from the restriction gap 37, so as to continuously clean the same 'in this fashion also. In addition, the rotary impeller 36 may be employed, as it is in FIGURE 1, as essentially the sole source of recirculation of liquor in the digester. It will be appreciated that under relatively high pressure conditions in the digester 11, 1112 or 110 (of FIGURES 2 or 3), in each case the tendency is for a relatively substantial stream of liquor to flow beneath each of these rotary impellers 36 in the so-called restricting devices R and out through the lines 34 to the headers a, b or c. This rather substantial stream of liquor will carry the freed-fibers rapidly away from the interior of the digester and thus rapidly away from the scene of maximum chemical activity, while at the same time these rotary impeller features of the exit restrictors R, R will also serve to agitate the chips in the liquor and to recirculate liquor :and otherwise carry out many of the other functions that are desired in the instant process and apparatus. It Will thus be seen that in the embodiment of FIGURE 1, the chips or fiber bundles or particles will remain in the digester 11 until they are reduced to freed-fiber size during this continuous circulation process in the area of maximum chemical activity and shortly after being reduced to the freed-fiber size the liquor separator 12.

prising a plurality of pump impeller type blades 36a, 7'

36b, 36c extending outwardly from a generally flat diskshaped bottom 36x for the impeller 36 to which the shaft 30 is fixed in rigid connection or, as here shown, integral with the head 36 (e.g. as a part of a simple casting). The rotary impeller head 36 serves to generally impart centrifugal force to chips, fiber bundles or similar solids dispersed in the liquor in the digester, as they tend to approach what is actually the substantially freed-fiber size restriction indicated generally at 37, which is an annular area of minimum thickness between the outer peripheral underside of the impeller disk element 36x and a generally annular rib 38 (shown in full view for convenience in FIGURE 4) that is mounted on the digester wall shown fragmentarily at 39. The annular rib 38 may, of course, be mounted on shims or otherwise adjustable so as to adjust the gap 37 and thus afford more precision to the concept of the substantially freed-fiber size of the resulting freed-fibers themselves will find their way through the corresponding gaps (indicated at 37 of FIG- URE 4) for each of the devices R, R and on into the liquor stream header 35a and then to the pressurized pulp- In the pulp-liquor separator 12, it has already been explained that the device used may be a conventional screw conveyor type of device operating within a strainer such that the pulp is pressed and the free liquor is expressed from the pulp and through the strainer back into a. liquor recycle line indicated at 40 in FIGURE 1.' The pressed pulp which has then had substantially 10% to of the free liquor physically separated therefrom exits from the screw press or other device 12 through a. line 41, while still under pressure, and into a trap tank 4-2 which is also maintained under pressure. It will be noted that the trap tank 42 has a vent line 43 "feeding to a main vent header 44 which also takes off via the line 45 from the top of the digester 11 so as to vent air, volatile matter such as terpentine and the 1 like which enter the system through the chips and which must be periodically vented to afford smooth continuous operation. The venting, however, to the header 44 may be controlled, for example, through a valve 46 so that there is not a total pressure loss in the digester 11 or from the digester 11 clear through to the trap tank 42, since it is desired to maintain a substantial amount of pressure ordinarily in the practice of the instant invention throughout this sequence of treatment for the cellulosic material. The cellulosic material entering the trap tank is in the form of relatively concentrated pulp, i.e. of relatively high consistency, but it is also at a relatively high temperature and immediate exposure to ambient atmosphere would result in a tendency to burst the fibers because of immediate volatilization of water contained within the fibers. In the practice of the invention means are provided at the various stations for handling the fibers such that the pressure at the immediate periphery of the fibers as well as the chips or other particles is sufficiently high at all times to preclude mechanical bursting of the same by release of internal pressures. This means, of course, that the pressure in the digester 11 is maintained'sufiiciently high to preclude any creation of vacuum or re;

duced pressure below the saturation pressure of the liquor in the particles at the off-running sides of, for examples, the impeller blades 36a, 36b, etc. This is done merely by maintenance of a sufficiently great predetermined overall pressure in the digester to compensate for any localized minimum pressure areas that may develop through the use of agitation means or the like; and this feature of control is not particularly difficult for the reason that the ratio of dry solids to liquor is quite low in the digester, ranging from about 1:100 to perhaps 1:8 and the liquid suspension in the digester is thus highly fluid and this highly fluid suspension in the digester can thus be subjected to the action of mild agitating devices and/or reasonably rapidly moving recirculating impellers or pump impellers without the generation of any significantly low pressure areas in the digester, where the temperature will also be substantially above ordinary ambient atmospheric temperature, in fact substantially above the boiling point of water in most instances such that release to ambient atmosphereof the matter in the digester would cause yol-atilization of water within the fibers and mechanical bursting thereof. It will thus be appreciated that a superatmospheric pressure is continuously maintained in the liquor suspension in the digester and in the stream carrying the freed-fibers in the lines 34 and the header 35a down to the pressurized pulpliquor separator 12 and through the pulp exit line 41 to the trap tank 42, wherein therelatively high consistency pulp is subjected initially to cooling Wash Water via the line 49 and this cooling wash water has the net effect of reducing the temperature throughout the body of the fibers of the pulp to a temperature below the volatilization temperature of the liquor (under the pressure conditions at the exit the valve 50) within the pulp fibers which are then drawn out of the bottom of the trap tank through a control valve 50 and into a line 51 feeding to the pulp washers. It will be appreciated that the control valve 50 is actuated by a liquor level control sensor LLC indicated diagrammatically sensing the liquor level control in the trap tank and thereby maintaining the same substantially constant under the desired conditions of operation. In this way a conventional pressure control is maintained in the trap tank and controlled release of liquid from the trap tank, in the form of the pulp going to the washers in the line 51 is effected at a temperature that is reduced by the wash water from the line 49 so as to avoid the possibility of mechanical bursting hereinbefore mentioned.

The foregoing essentially describes the forward progress of the cellulosic component through the system- 10. It has already been explained that the forward progress out of the digester does not take place until freedfibers are produced in the digester, so there is a recycling of chips and particles and the like in the digester but otherwise there is continuous forward progress of the freed cellulosic fibers in the manner just described.

The so-called free fiber size restrictors R, R, which constitute essentially re-pulper type agitators mounted with a discharge concentric with the drive shaft therefrom and behind the non-agitating side of the so-called agitator R, constitute devices which, of course, are shown in FIGURE 4, but which per se have the characteristic of commercially available individual units (i.e. the Morden Slushmaker) agitator-pulper type with adjustable gap pulp outlet behind the agitator.

Referring now to the main liquor cycle, it has already been pointed out that a substantial stream of liquor will pass behind the agitators in the devices R, R through the outlets 34, 34 into the pulp and liquor header 35a (carrying essentially free-fibers) through the physical separation zone or device 12 and, therein, the liquor to a substantial extent is separated from the pulp and the pulp of increased consistency is driven from the physical separator through the line 41 into the trap tank 42, whereas the separated free liquor is withdrawn through the recirculating line 40 feeding into a recirculating system header 60 which in turn feeds into a recirculating pump 61. It

will be noted that a conventional pH control device is indicated diagrammatically as sensing the pH in the suction recirculation header 60 to the recirculating pump 61 and controlling a valve 63 for controlled admission of fresh liquor from the fresh liquor supply 64 into the suction of the circulating pump via the header 60. It will be appreciated that there is a consumption of chemicals of whatever type may be used in the fresh liquor during the digestion and pulping operations and this consumption, for example, on the basis of total dry fiber Weight, may be approximately 14 to 18% by weight of the active alkali employed (ie the sodium oxide equivalent, e.g.) in the case of sodium hydroxide alkali. Accordingly, the chemicals thus consumed in the operation must be returned to the system via the recirculation pump 61 and it is preferable to first dilute the chemicals, in the form of fresh liquor from the source 64, with the bulk of the recirculating liquor in the header 60 so that localized high concentration of chemicals will not occur in the digester.

The liquor continues from the discharge of the recirculating pump 61 through a regulating valve 65, a conventional heating source 66, which is here shown as a conventional heat exchanger in which steam is fed controllably into the so-called outside phase of the exchanger 66 and condensate is removed therefrom via the lines 67, 67 in conventional manner and direct contact steam heating is not employed, since the liquor will pass through the inside phase or pass of the heat exchanger 66. Heating of this type is not an absolute necessity but the versatility of the invention is demonstrated in that heat may be returned to the system in this way without the necessity of adding water thereto, inthe event that such is desired. The circulating pump 61 thus provides a discharge line 68 feeding through the control valve 65, the heat exchanger 66 and into the digester 11 to substantially complete the cycle of the liquor system, which cycle is for practical purposes adiabatic, in that it is desired essentially to retain all of the heat of the liquor in the digester, within the limits of practicability, rather than withdrawing liquor from the digester and cooling the same and then having to' reheat the same deliberately. Of course the fresh liquor from the fresh liquor source 61 will have a cooling effect and some makeup heat, for example, via the heat exchanger 66 would be required to compensate for the fresh liquor 64 and also there will be a certain amount of unavoidable heat loss in the recycle system, no matter how effective the insulation of the conduits 40, 60, 68, etc. may be. The essential purpose in the various heat adding devices is to heat the chips (being added) to the desired cooking or digester conditions. Other conventional heat recovery systems may be used also.

It will also be noted that an additional control is afforded again on the basis of conventional equipment, but in this case the control constitutes a consistency sensing device 69 of conventional construction in the pulp-liquor discharge header 35 from the digester 11, which transmits a signal (indicated via the dotted line 69a) to the control valve 65 to control the rate of recirculating liquor, in such a manner that the total amount of recirculating liquor will adequately maintain a pre* determined consistency in the header 35a, which consistency may vary to suit the particular needs but is preferably substantially within the previous low consistency range suggested for the digester itself. It must be appreciated, however, that a substantial amount of the liquor in the recycle stream actually does pass through the header 35a and carries with it the freed-fibers so that even lower consistencies than the consistency previously set forth as a range for the digester may result in the pulp-liquor header 35a. The consistency actually will be that suiting the particular convenience and needs of the operator for effectively maintaining the restriction gap so that the restrictors R are free and clear and keep-- ing the progress of freed-fibers out of the digester at the desired rate. To do this in any given system requires a certain amount of liquor as carrier and this amount will be readily determined in any given situation by the skilled operator and once determined it may readily be used in the control system 69, 69a, 65 hereinbefore described.

In addition, the digester 11 will have a liquid level control, indicated diagrammatically in the box LLC sensing the level L-ll in the digester 11 through the sensing line indicated diagrammatically at 71 and controlling through a conventional control line indicated diagrammatically at 72 a liquor control valve 73 for withdrawing liquor from the recirculating system via a liquor recovery line 74 taking off from the discharge line 40 from the physical separating zone 12. Thus, it will be appreciated that a continuous buildup of total liquor or water content in the digester cannot be permitted and a certain amount of spent liquor must be withdrawn through the line 74 via the control valve 73 continuously during operation to compensate for makeup fresh liquor from the source 64 and otherwise adequately maintain the liquid level L-l'in-the digester 11. The liquor recovery system to which the line 74 feeds is conventional and need not be described in further detail. It will be noted, however, that there is shown a return line 75 from the pulp washers (also conventional and not shown) which may return a certain amount of spent liquor to the liquor recovery header 74 for the same purpose. The entire liquor system is thus explained.

With respect to the heat balance of the system, it has already been explained that there will be used a substantially greater volume of liquor, with a substantial amount of heat content in the digester 11, so that the rather substantial volume of liquor employed in the digester 11 in order to obtain the desired fluidity will necessarily have to flow through the cycle 35a, 40, 60, 68 and back to the digester 11 in a manner which will necessarily involve certain inherent heat losses, although all such heat losses are avoided within the limits of practicability so as to have an essentially adiabatic system with respect to the truly recycled liquor. Heat losses will occur in the pulp removed from the physical separating zone 12!) of the line 41, in the liquor sent to the recovery system via the header 74, and in the fresh liquor addition via the control'valve 63 (which is not technically a heat loss but rather an inadequate heat addition since the material added is not hot enough), and heat addition to the liquor system can take either or both of two forms. One of these is the so-called dry heat system 66 where live steam is not permitted to enter into the liquor itself in the heat exchanger 66 and the other is indicated diagrammatically at the line 25 wherein live steam is fed from a steam header 76 through a control valve 77 and directly into the digester 11 in response to temperature control signals taken via the line 78a to the signal receiver indicated diagrammatically at 78 which controls the valve 77 via the signal line 78b, all of which is a conventional temperature control system and is shown diagrammatically to simplify the disclosure.

Although it is appreciated that each set of conditions will cause variations in the overall operation which are well within the skill of the operator, a typical set of conditions Will be described, as follows:

Since the chips C are fed through the pressure feeder 21 directly into the digester 11, there will be a nominal unknown with respect to the time-temperature-chemical reaction relationship while the chips pick up enough moisture to enter into the cycle, but this so-called unknown will actually turn out to be a predetermined nominal number of minutes, in the neighborhood of about five minutes on the average. The wood chips used are of conventional size and character in this particular instance for kraft pulping. The entrapped air and noncondensible gases entering therewith will be volatilized quite rapidly upon reaching the hot liquor in the digester and this volatiliza- L) tion will create a pressure head via the back pressure on the vent control valve 46 so that a control pressure of 140 pounds per square inch gauge is maintained on the digester, on the trap tank 42 and on the system intermediate the same. Using a dry fiber to liquor ratio of 1:20 in the digester the temperature controller is set for 340 F. The incoming raw steam through the control valve 77 is set to maintain the aforesaid temperature controllably. The consistency sensor 69 is set to control the recirculating liquor valve 65 to substantially maintain the previously specified consistency for the interior of the digester, in conjunction with a predetermined level setting through the digester level control LLC. The alkali consumption set for the particular system is computed on the basis of 16% of the dry fiber weight and the sulfidity in the cooking liquor in the digester 11 is maintained at in this particular example, which involves essentially control of the entrance of fresh liquor via the pH controller 62. The fresh liquor in the source 64 contains a concentration of sodium sulfide and alkali such that not only the predetermined sulfidity in the digester 11 is maintained, but the scene of maximum chemical activity in the digester 11 is maintained such that the effective alkali concentration therein is grams per liter.

perature conditions in the digester.

Of course, the rate of rotation of the impellers at the restriction gaps R, R will effect the total rate of recirculation in the digester itself as the material is continuously circulated in cycles therein and the peripheral fibers are continuously subjected to the scene of maximum chemical action, have their bonds continuously loosened and are ultimately themselves continuously freed and then after a minimum average dwell time in the digester are caused to flow out through the restrictors R, R and into the header 35a. The speed of the liquor press transfer system for the physical separation zone 12 is controlled so that an amount of additional equalizing cooking may take place in the pressurized pulp-liquor separation zone which is operated at substantially the same pressure as that of the digester, and substantially the same temperature in order to minimize heat losses in the overall recirculating system.

It will be appreciated that the chemical cooking of the free-fibers is decelerated rapidly by the addition of wash water via the line 49 into the trap tank and by the subsequent pulp washers, so the actual cooking time here involved is the total time in the digester 11, the header 35a and substantially that in the pressurized pulp-liquid separator 12, with a slight additional cooking time in the trap tank during the cooking deceleration which is effected by the result of the cooling effect of the wash water therein. The average time for chips to retain their particle condition- (i.e. before they are reduced to the freed-fiber size which will pass the restrictors R) during the continuous recycle in the digester in the scene of maximum chemical action is computed on the basis of fifteen minutes in the present system and the dwell time of the freed-fibers from the restrictors to the trap tank 42 is computed on the basis of a dwell time of substantially five minutes, which results in what constitutes a total cooking and digestion time of only twenty minutes in the computation of the specific example here involved.

It must be appreciated that from the theoretical point of view the peripheral fiber on a given chip will undergo only nominal response to the chemicals in the scene of maximum chemical action, in the digester 11, under conditions which involve only the theoretical exposure of the exterior of the peripheral fiber to the scene of maximum chemical action with the interior bonding matter, i.e., the lignins, protecting the back side of the fiber in a nominal 9 fashon so that, theoretically, in the concept of chemical cooking gradient here involved the exterior of the fiber itself is really being cooked faster than the interior, but it is ultimately the cooking of the interior side of the fiber that results in the delignification which weakens the bond and permits the mild agitation to free this fiber. Once the fiber is freed, then it exists momentarily in the scene of maximum chemical action in the digester under conditions of maximum effective chemical attack. This period of time is preferably a period of time which is minimized by the recirculation rate within the digester so that there is a net effect of a theoretical recirculation rate such as that which would effectively wash away the freedfibers through the restrictors R in an average time of perhaps about one minute dwell time per fiber. It is conceded that this type of computation is essentially theoretical, but an abstract approach is believed to be helpful I in some understanding of the nature of the invention. The basic concept is that the so-called ligin filled fiber will remain adhering to the chip and thus be partially protected from maximum chemical attack until the delignification of this particular individual fiber is substantially effected to the extent that it breaks loose from the particle or chip. Up to that moment, only a portion of the exposed side of the fiber is under maximum chemical attack and even a much more nominal portion of the immediately interior fiber is under direct maximum chemical attack. Once the fiber is freed, however, its finds itself in an environment of maximum chemical activity which, in order to create the desired chemical action gradient from exterior to interior of the chips and particles, may be stronger chemical action than would be used in the batch or other slow speed types of digestion and pulping, so-it is imperative that the recirculation ratebe such that the dwell time of the freed-fiber is diminished to a minimum amount of time and it is rapidly passed on through the restrictors R and into the pressurzed pulp-liquid separaton. Obviously, the movement of the freed-fiber in the digestion liquor through the header 35a can be accomplished in a matter of almost seconds so that cooking effect therein is theoretically negligible, but in the physical separating zone 12 there Will again be a measurable dwell time during which the pulp will at least initially be exposed to chemical concentrations, temperature and pressure substantially identical to that in the scene of maximum chemical activity in the digester.

Again, however, it must be noted that the physical separation system involving a liquor transfer press or some other typeof device which relatively rapidly separates a substantial quantity of the liquor from the fibers will thus start an immediate but controlled deceleration of maximum chemical action in that it will immediately start to decrease the relative concentration of chemical to dry fiber, even though the pressure and temperature may not be altered significantly at this point. The relative decrease in chemical concentration to dry fiber is effected under controlled conditions, in fact, readily controlled conditions since the operation of transfer presses or other pressurized pulp-liquid separators is a comparatively easily controlled operation and the relationship between chemicals present and dry fiber present can be varied under controlled conditions over a predetermined period of time, such as the five minutes here proposed, so that the exact amount of actual chemical cooking which takes place in this physical separation zone may be assumed to be controllable on this basis, even assuming that a substantially constant temperature and pressure is being maintained on the individual fibers during this controlled deceleration of chemical cooking. It will further be appreciated that in the physical separation zone there will be a nominal number of fibers which have probably already been overcooked and a nominal number of fibers which have probably been freed overly rapidly through accident or otherwise in the digester and are substantially undercooked, but the great bulk of fibers in the pulp in the physical separation zone 12 will be in a relatively limited range of so-called cooked conditions which may Well be measurable by the average lignin content. Thus under controlled conditions the average lignin content entering the pulp physical separation zone 12 will in the particular embodiment here involved be substantially 18% plus or minus about 1%, for about 90% of the fibers. This is a feature of the invention in that the average amount of dwell time of a freed-fiber in the scene of maximum chemical activity is relatively controllable by the use of the rapidly moving liquor cycle and the restrictors for removing such freed-fibers in the manner just described. If it is desired then to reduce this approximately 18% lignin content averaging in some 90%of the fibers in the physical separation zone 12, it will be appreciated that the speed of the liquor transfer presses or other devices employed may be controlled and, for example, if it is desired to decrease this lignin content only 2% on the average, then the dwell time in the physical separation zone can be held at perhaps five minutes, but if it is desired to decrease the lignin content, for example, 5%, it will be appreciated that the dwell time in the physical separation zone 12 can be controlled so that it is a correspondingly longer period of time. There is an equalizing effect in connection with the physical separation zone or system, also, in that fibers having maximum lignin contents will tend to be more receptive of chemical action than fibers having minimum lignin contents in this system (more or less as a corollary of the so-called law of mass action in chemistry), since many of the variables which would alter the'tendency for maximum lignin concentrationsto be subjected to maximum chemical attack are eliminated. Thus there are substantially no localized high pressure or high temperature spots in these physical separation zones,.the actual relationship of chemical content to dry fiber content is controllable on almost an instantaneous basis throughout the entire physical separation process, and the essential feature of the restrictor-scparation system from the digester has the net effect of introducing substantially only free fibers to the pressurized separation zone 12 in substantially a uniform physical condition of size, fibrillation, saturation, etc. so that the conditions for chemical equalization of the liguin content are most favorable.

Referring now to FIGURE 2, as an alternative, it may be assumed that essentially the same operating conditions in connection with time-temperature concentrations pressure, etc. are employed in the operation of the digester 11a which is substantially identical instructure to the digester 11 except that preferably the restrictors R, R are positioned near the bottom thereof, again so that localized dead spots cannot be created, but directly in the path of a substantial stream of recirculating liquor which flows directly past the restrictors R, R and out the bottom of the digester through a suction line into a simple recirculating pump 81 which returns the liquor through a very short cycle 82 back into the digester 1121 at a predetermined submerged level indicated at the arrow at 82a so as to afford an additional control variable in the system. The four restrictors R, R; will again take off a substantial stream of liquor containing freed-fibers into the header 35b for physical separation at 12, as indicated in FIGURE 1, but the recirculation system 80, 81, 82 is intended to afford an additional control to the actual dwell time of freed-fibers within the digester 11a.

It must be appreciated that the operation of the digester 11 or 11a, or llbof FIGURE 3, is not merely that of washing a substantial body of chips with a relatively minute amount of liquor so that loose fibers would be Washed off and their dwell time in the digester could be accurately computed solely on the basis of the dwell time of recirculating liquor passing through the digester, since an important feature of the instant invention involves the use of such substantial quantities of liquor that we have an essentially continuous phase of liquor with a dispersed phase of chips therein and the liquor dispersion has the high fluidity desired. This being the case there is always the possibility that a certain number of freed-fibers will becomt involved in the recirculation rather than the drawolf system and this is unavoidable to some extent. On the other hand, by selective control of the recirculation systems and the speed of liquor circulation through the di gester via one or more streams, it is possible to cause sufiiciently rapid movement of a substantial mass of the liquor in the digester through the size restricting gaps R, R so that the substantial majority of freedfibers will have a very short average dwell time in the digester 11, 11:! or 11!), which we have already explained is the scene of maximum chemical activity. An important concept of the instant invention involves not only the creation (rather than the heretofore suggested destruction) of the reaction gradient from exterior to interior of the given chip in the scene of maximum chemical action, but the minimizing of the dwell time of the fiber once freed in this scene of maximum chemical action. This is accomplished simply by controlled flow speeds of the recirculating liquor streams.

Referring to FIGURE 3, it willbe seen that a somewhat different arrangement is shown, although the digester itself 11b is constructed essentially like the digester 11a with the restrictors R, R positioned at the bottom thereof near a discharge header 80b feeding into the suction of a recirculating pump 81b which in turn discharges through a short recirculation line 82b back into the digester 11b for a so-called short circuit recirculation. Again, the fundamental liquor recirculation via the physical separation zone 12 and the liquor system 40, 60, 68, etc. of FIGURE 1 is still employed, but the devices of FIGURES 2 and 3 involve an additional control of the actual fluid speed of liquor moving through the digester 11a or 11b.

One additional feature of the system of FIGURE 3 involves a pre-wetting of the chips C fed in at 90 which are prewetted with white liquor fed in at 91 and then in this pre-wet mixture pressurized and fed through a transfer press 92 into the recirculating pump 81b, suction 80a via a conventional screw 92a in the transfer press, actuated by a motor 92b and provided with the conventional cylindrical screen liquor discharge system 93 which affords release of excess white liquor and recirculation back through the line 91 for further makeup with the chips 90. This system affords certain advantages in that the chips are actually fed into the short circuit cycle 80!), 81b, 82b at substantially the point at which particles initiate their movement through the cycle after they have just released their exterior cover of peripheral fibers at the approximate region immediate to the restrictor gap devices R, R. Thus, the particles which have just developed newly exposed peripheral fibers are mixed with wet chips which also provide fibers which are in eifect the newly exposed peripheral fibers comparable to those of the particles actually in the recycle process.

Expressed process-wise, it will thus be seen that the system with or without the alternative embodiments of FIGURES 2 and 3 incorpoarted therein involves a continuous digester-chemical pulping method, which comprises 1) continuously driving a pulping liquor stream at a predetermined pressure and cooking temperature through a cycle consisting essentially of a digester zone feeding through a discharge zone into a physical separation zone and then back to said digester zone; (2) introducing digestion liquor and lignocellulose fiber particles to said digester to continuously maintain a highly fluid suspension in said digester having substantially a dry solids to liquor ratio of 1:100 to 1:8; (3) continuously maintaining in said digester zone a chemical digestion and de-lignification concentration-temperaturepressure relationship sufficient to continuously effect chemical de-lignification with resultant weakening of the bond of peripheral fibers of said particles at a substantially greater rate than with internal fibers of such particles; (4) continuously subjecting-the suspension to mild agitation sufficient to continuously free from such particles individual peripheral fibers whose bonds are so weakened, thereby exposing new peripheral particle fibers to the conditions of (3); (5) continuously driving said liquor stream bearing the aforesaid freed-fibers of (4) through a substantially free-fiber size restriction in said discharge zone and into said physical separation zone where a substantially free-fiber size restriction in said discharge Zone and into said physical separation zone where a substantial portion in the range of 10% to 95% of the free liquor is physically separated from said free-fibers and recycled back to said digester zone in the cycle at (1); (6) continuously subjecting the aforesaid particles to movement in the dispersion in the immediate vicinity of the oncoming side of said restriction to effect movement of the same countercurrent to the fiber-bearing liquor stream entering said restriction; and during the aforesaid steps (1) through (6) continuously maintaining pressure at the immediate periphery of said particles and fibers sufiiciently high to preclude mechanical bursting of the same by release of internal pressure and while also continuously maintaining mechanical agitation sufiiciently mild to preclude substantial mechanical fiber degradation; (7) subjecting the physically separated freefibers and liquor retained thereby to continued conditions of predetermined pressure and chemical cooking to reduce the lignin content to below the average content of free-fibers leaving the digester zone; and (8) then quenching the product of step 7) to substantially reduce the internal fiber temperatures thereof to below C. to permit rapidly reducing the pressure thereof to ambient at mospheric pressure.

It will be appreciated that in a typical system the process step (3) involves temperatures within the range of about 100 to 200 C. and superatmospheric pressures within the range of about 20 to pounds per square inch gauge. Additionally, in the kraft process the alkali concentrations should be about 5 to 25% of the dry fiber weight on the basis of the 100% sodium hydroxide concentration, and about 10% to about 50% sulfidity in the digester itself. In other processes the pressure and temperature are conveniently within the ranges hereinbefore stated, but other chemicals such as combined alkali-S0 are used in relatively high concentrations, or merely the alkali may be used in the concentrations hereinbefore specified as a range.

In general, it Will be appreciated that the combination of temperature-pressure-chemical concentration should be such that the gradient of reaction from exterior to interior of the individual chips and/ or particles is created. This will necessarily involve in most instances a reaction condition relationship between these variables which effectively exceeds the digestion-pulping batch speeds by at least about 25% on the basis of chemical action and preferably by at least about several hundred percent, so that the total dwell time of freed-fibers at least in the scene of maximum chemical action is reduced to a minute fraction of that contemplated in previously considered batch processes. Essentially, the definition of the invention in this respect involves chemical cooking or de-lignification at such a rate that the weakening of the bonds of peripheral fibers is substantially accelerated and conspicuously faster than the weakening of the bonds of the interior fibers. This concept is hardly capable of simple numerical expression, although it is easily appreciated by the skilled worker in the art, since it is essentially an exaggeration or amplification of a chemical cooking result which the skilled worker in the art has sought to avoid for years and it is coupled with the concept of recirculating excessive amounts of liquor (from the prior art point of view) through a restricted gap zone so as to reject unfreed-fibers but withdraw freed-fibers very rapidly from the scene of maximum chemical action. Such rejects are of course actually recirculated and they are recirculated very rapidly preferably in the process of the invention because the chemical gradient acting thereon is such that they will very rapidly shed another peripheral coating of fibers in the immediate future and this newly shed peripheral coating of freed-fibers must then be removed from the scene of maximum chemical action with substantially the speed (i.e. after substantially the dwell time) that is employed in freeing and removing the first peripheral layer of fibers on any given chip or particle fed into the system.

It will also be appreciated that the instant invention does comprise an apparatus systemin the sense that it comprises a combination of [1163115,311 hereinbefore described in detail, but which are described in a general sense as a continuous digester-chemical pulping system, which comprises (1) means for continuously driving a pulping liquor stream at super-atmospheric pressure and cooking temperature through a cycle consisting essentially of a digester zone feeding through a discharge zone into a physical separation zone and then back to said digester zone; (2) means for continuously introducing digestion liquor and ligno-cellulose fiber particles to said dig-ester to continuously maintain a highly fluid suspension in said digester having substantially a dry solids to liquor ratio of 1:100 to 1:8; (3) means for continuously maintaining in said digester zone a chemical digestion and de-lignification concentration-temperature-pressure relationship sutficient to continuously effect chemical de-lignification with resultant weakening of the bond of peripheral fibers of said particles at a substantially greater rate than with internal fibers of such particles; (4) means for continuously subjecting the suspension to mild agitation sufficient to continuously free from such particles individual peripheral fibers whose bonds are so weakened, thereby exposing new peripheral particle fibers to the conditions of (3); (5) means providing a substantially free-fiber size restriction in the discharge zone between relatively moving surfaces for dividing the liquor stream into a first and a second stream; (6) means continuously driving the first stream bearing freed-fibers of (4) through such restriction and into said physical separation zone where a substantial portion in the range of to 95% of the free liquor present is physically separated from said free fibers and recycled back to said digest-er zone into the cycle at (1) under substantially abiabatic conditions; (7) means continuously driving particles not yet reduced to freedfibers away from such restriction in the second stream, thereby continuously cleaning such restriction, and directly back into the cycle at (1) also under substantially adiabatic conditions; and (8) means operatively connected with said means (1) through (7) for continuously maintaining a pressure-temperature relationship at the immediate periphery of said particles and fibers in the aforesaid means (1) through (6) such that at all times and locations in said system the minimum pressure areas are above the saturation pressure of the liquor in the particles and fibers, thereby precluding mechanical bursting of the fibers by release of internal pressures.

It will be understood that modifications and variations may be efiected without departing from the spirit and scope of the novel concepts of the present invention.

1 claim as my invention:

1. A continuous digester'chemical pulping which comprises (1) means for continuously driving a pulping liquor stream at superatmospheric pressure and cooking temperature through a cycle consisting essentially of a digester zone feeding through a discharge zone into a physical separation zone and then back to said digester zone;

(2) means for continuously introducing digestion liquor and ligno-cellulose fiber particles to said digester to continuously maintain a highly fluid suspension in said digester having substantially a dry solids to liqu-or ratio of 1:100 to 1:8;

(3) means operatively connected with means (1) system,

through (5) hereof for continuously maintaining a pressure-temperature relationship at the immediate periphery of said particles and fibers in such means (1) through (5) such thatat all times and locations in said system the minimum pressure areas are above the saturation pressure of the liquor in the particles and fibers, thereby precluding mechanical bursting of the fibers by release of internal pressures; said means (3) also includingme-ans for continuously maintainingin said digester zone a chemical digestion and de-lignification f concet tration-temperature-pressure relationship sufficient to continuously effect I chemical de-lignification with resultant weakening of the bond of peripheral fibers of said particles at a substantially greater rate than with internal fibers of such particles; r r I (4) rotary blade means for continuously subjecting the suspension to mild agitation sufiicient to continuously free from such particles individual peripheral fibers whose bonds are so weakened, thereby exposing new peripheral particle fibers to the conditions of (3); said rotary blade means (4) being positioned within the digester for continuously subjecting the aforesaid particles to movement in the dispersion in the immediate vicinity of the oncoming side of the restriction of means (5) to effect movement of the same countercurrent to the fiber-bearing liquor stream entering such restriction and to continuously clean such discharge restriction; and

(5) pressure differential creating means for continuously driving said liquor stream bearing the aforesaid freed fibers of (4) through a substantially free-fiber size restriction defined by spacing between first and second substantially coextensive surface portions, one of which is :held against rotation and the other of which is rotated relative to said one, in said discharge zone outside of the digester and into said physical separation zone where a substantial portion in the range of 10% to of the free liquor is physically separated from said free fibers and recycled back to said digester zone in the cycle at (1).

2. A continuous digester-chemical pulping system,

which comprises (1) means for continuously driving a pulping liquor stream at superatmospheric pressure and cooking temperature through a cycle consisting essentially of a digester zone feeding through a discharge zone into a physical separation zone and then back to said digester zone;

(2) means for continuously introducing digestion liquor and ligno-cellulose fiber particles to said digester to continuously maintain a highly fluid suspension in said digester having substantially a dry solids to liquor ratio of 1:100 to 1:8;

(3) means operatively connected with the means (1) through (6) hereof for continuously maintaining a pressure-temperature relationship at the immediate periphery of said particles and fibers in such means (1) through (6) such that at all times and locations in said system the minimum pressure areas are above the saturation pressure of the liquor in the particles and fibers, thereby precluding mechanical bursting of the fibers by release of internal pressures; said means (3) also including means for continuously maintaining in said digester zone a chemical digestion and de-lignification concentration-ternperature-pressure relationship sufiicient to continuously effect chemical de-lignification with resultant weakening of the bond of peripheral fibers of said particles at a substantially greater rate than with internal fibers of such particles;

(4) rotary blade means for continuously subjecting the suspension to mild agitation sutficient to continuously free from such particles individual peripheral fibers whose bonds are so weakened, thereby expos- (5) means providing a substantially free-fiber size restriction in the discharge zone between relatively moving surfaces within the digester and adjacent an interior wall thereof for dividing the liquor stream into a first and a second stream;

(6) pressure differential creating means continuously driving the first stream bearing freed fibers of (4) through such restriction and into said physical separation zone where a substantial portion in the range of 10% to 95% of the free liquor present is physically separated from said free fibers and recycled back to said digester zone into the cycle at (1) under substantially adiabatic conditions.

3. The system of claim 1 wherein the restriction de- 16 fined in means (5) consists of the aforesaid one of said surface portions held against rotation which is mounted within the digester and the other of said surface portions mounting the aforesaid rotary blade means (4).

4. The system of claim 2 wherein the aforesaid restriction of means (5) consists of means held against rotation Within the digester for defining one of the aforesaid relatively moving surfaces, with the other of said relatively moving surfaces being presented by said rotary blade means (4).

References Cited by the Examiner UNITED STATES PATENTS 2,882,967 4/1959 Surino 162-246 FOREIGN PATENTS 25,896 7/1953 Finland.

DONALL H. SYLVESTER, Primary Examiner.

20 s. LEON BASHORE, Examiner. 

1. A CONTINUOUS DIGESTER-CHEMICAL PULPING SYSTEM, WHICH COMPRISES (1) MEANS FOR CONTINUOUSLY DRIVING PULPING LIQUOR STREAM AT SUPERATMOSPHERIC PRESSURE AND COOKING TEMPERATURE THROUGH A CYCLE CONSISTING ESSENTIALLY OF A DIGESTER ZONE FEEDING THROUGH A DISCHARGE ZONE INTO A PHYSICAL SEPARATION ZONE AND THEN BACK TO SAID DIGESTER ZONE; (2) MEANS FOR CONTINUOUSLY INTRODUCING DIGESTION LIQUOR AND LIGNO-CELLULOSE PARTICLES TO SAID DIGESTER TO CONTINUOUSLY MAINTAIN A HIGHLY FLUID SUSPENSION IN SAID DIGESTER HAVING SUBSTANTIALLY A DRY SOLIDS TO LIQUOR RATIO OF 1:100 TO 1:8, (3) MEANS OPERATIVELY CONNECTED WITH MEANS (1) THROUGH (2) HEREOF FOR CONTINUOUSLY MAINTAINING A PRESSURE-TEMPERATURE RELATIONSHIP AT THE IMMEDIATE PERIPHERY OF SAID PARTICLES AND FIBERS IN SUCH MEANS (1) THROUGH (5) SUCH THAT AT ALL TIMES AND LOCATIONS IN SAID SYSTEM THE MINUMUM PRESSURE AREAS ARE ABOVE THE SATURATION PRESSURE OF THE LIQOUR IN THE PARTICLES AND FIBERS, THEREBY PRECLUDING MECHANICAL BURSTING OF THE FIBERS BY RELEASE OF INTERNAL PRESSURES; SAID MEANS (3) ALSO INCLUDING MEANS FOR CONTINUOUSLY MAINTAINING IN SAID DIGESTER ZONE A CHEMICAL DIGESTION AND DE-LINGNIFICATION CONCENTRATION-TEMPERATURE-PRESSURE RELATIONSHIP SUFFICIENT TO CONTINUOUSLY EFFECT CHEMICAL DE-LIGNIFICATION WITH RESULTANT WEAKENING OF THE BOND OF PERIPHERAL FIBERS OF SAID PARTICLES AT A SUBSTANTIALLY GREATER RATE THAN WITH INTERNAL FIBERS OF SUCH PARTICLES; (4) ROTARY BLADE MEANS FOR CONTINUOUSLY SUBJECTING THE SUSPENSION TO MILD AGITATION SUFFICIENT TO CONTINUOUSLY FREE FROM SUCH PARTICLES INDIVIDUAL PERIPHERAL FIBERS 